The CB Radio Talk Forum

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The CB Radio Talk Forum

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are those springs really 6 inches or just like 4??i figure once u buy the spring and 102" from radioshack, after all the taxes u spend close to 38$$..i figured, might as well just get a real 11 meter quarter wave instead of a 102" with what acts like a loading coil at the bottom, they work good though

Two questions:
1. Why would a shorted spring turn into a loading coil?
A. The shorting strap through the center of it is broken.
(I've never seen one broken like that. Doesn't mean it can't be done, but I've never seen one in over 40 years.)

Length of 'whip' mounted on ball-mount and spring = 95.29 inches. A 102" whip is 6 and 11/16th inches too long for resonance on channel 19.
Not everyone uses a ball-mount, so change that 4" to whatever height the mount is.
Not everyone uses a spring, so if no spring, don't subtract 4" for the spring height.
-IF- your mount is 1.29" tall, then a 102" whip is just right.
- 'Doc

(I'm using 'Radio Shack' batteries in my calculator so check my math, it may be slightly 'off'. , I like that one.)

WOW.....howcome the swr with 102 is high and with 108 is flat???11 meter full wave length is 36 feet....36 feet divided by 4 is 9 feet....108 inches divided by 12 equals 9 feet...which is a full quarter wavelength...i thought that was an easier way of figuring it...ur right i forgot there was a strap inside the spring, so what your saying is im gonna have to cut that 108 down a little but because the antenna starts at the feedline??im not gonna have a ball

'Doc wrote:Two questions:
1. Why would a shorted spring turn into a loading coil?
A. The shorting strap through the center of it is broken.
(I've never seen one broken like that. Doesn't mean it can't be done, but I've never seen one in over 40 years.)

Length of 'whip' mounted on ball-mount and spring = 95.29 inches. A 102" whip is 6 and 11/16th inches too long for resonance on channel 19.
Not everyone uses a ball-mount, so change that 4" to whatever height the mount is.
Not everyone uses a spring, so if no spring, don't subtract 4" for the spring height.-IF- your mount is 1.29" tall, then a 102" whip is just right. - 'Doc

(I'm using 'Radio Shack' batteries in my calculator so check my math, it may be slightly 'off'. , I like that one.)

Well that must be why my 102" screwed into my Predator 10K mount that is about 1.29" tall tunes flat?
Guess I won't mess with a spring

The calculations Doc gave are correct for wave length through the air. To say that a 1/4 wave length for 11 meter band is 9'...is not right on the money.

Through the air wave length will not be the same as the wave length through a the metal of an antenna. RF generally travels a bit faster through metal and the type and size (diameter) of that metal also has an effect. That is why no single "Rule of thumb" calculation will be perfect. The one Doc gave will get you the approximate ideal antenna length to START with. That length will usually be a little long and that is where you start to play with tuning to resonance.

Like I said above, I have a 102" SS whip attached to a 6" spring (with a center strap) and that is attached to a ball mount. Total length is about 109" from where the center conductor of the coax is attached. On my setup, I get SWR of about 1.2:1 to 1.3:1 across the band. In my book, that is pretty darn good!

Ooo, I really hate to say it, but an electron travels faster through a vacuum than through air, and faster through air than metal (or any solid). Since an electron travels faster through a vacuum than air, the length of a wave length in a vacuum is longer than a wave length through air. A wave length through metal/solids is shorter than the wave length through air. So, a fractional wave length through a metal would be shorter than the same fractional wave length through air/vacuum.

To make is even 'nastier', it deals with 'velocity factor', the difference between the speed of an electron through different mediums when compared to a vacuum.

That velocity factor is exactly the inverse of that of sound. Sound is faster through a more denser medium, slower though a less dense medium. (Bad comparison, but I don't know what else to use as a comparison.)